Radiation curable acrylic oligomers are usually comparatively high viscosity species and require to be blended with a monomer in order to produce a radiation curable formulation of the appropriate viscosity for any particular application. Such oligomers generally fall into three broad groups of resin, namely epoxy-acrylates, polyester acrylates and polyurethane acrylates.
The epoxy-acrylates include the B-hydroxy esters which are generated by the reaction of acrylic acid or methacrylic acid with an epoxy resin or epoxy-novolak resin. The polyester acrylates consist of polyesters which have been esterified with acrylic acid to yield a polyester with acrylate ester terminal groups, using well established esterification techniques. The polyurethane acrylates consist of reaction products of a hydroxy-containing acrylate ester, usually 2-hydroxyethyl acrylate or hydroxy propyl acrylate with an isocyanate prepolymer.
The monomers which are blended with the above acrylic oligomers in order to yield a practical radiation curable formulation in the presence of a suitable photoinitiator fall into three groups defined by functionality, and may be mono-, di- or poly-functional.
Poly-functional monomers, usually with a functionality of 3 or 4, generally consists of acrylate esters of tri or tetra-functional alcohols. Commonly used materials include glycerol triacrylate, trimethylol propane triacrylate, trimethylol ethane triacrylate, pentaerythritol tetracrylate, together with the acrylates of the ethoxylates or propoxylates of the above alcohols.
Di-functional monomers consist usually of the acrylate esters of ethylene glycol or propylene glycol and their oligomers, with tripropylene glycol diacrylate being especially preferred, diacrylates of longer chain alcohols such as hexanediol diacrylate and acrylate esters of cycloaliphatic diols such as the cyclohexane diols.
Mono functional monomers consist of the acrylate esters of mono functional alcohols such as octanol, nonanol, decanol, dodecanol, tri decanol and hexadecanol both in their linear and branch chain forms. Also included are cyclohexyl acrylate and its alkyl derivatives such as t-butyl cyclohexyl acrylate, tetrahydro furfuryl acrylate. N-vinylpyrrolidone has also been used as a mono-functional monomer. Styrene is used in certain formulations but is not widely used in this technology due to triplet quenching.
High functionality monomers give rapid cure speeds and high crosslink density, leading to films of high hardness and tensile strength with excellent chemical resistance. The films however suffer from reduced adhesion. Such monomers exhibit comparatively poor ability to reduce the working viscosity of the oligomer, due to the higher initial viscosity associated with the monomers themselves. Mono functional monomers, conversely, give slow cure speeds and low cross-link density, leading to cured films of lower hardness, tensile strength, and with reduced chemical resistance. Such monomers give films with improved elongation and improved adhesion, and the monomers show a considerably increased capacity to reduce the working viscosity of the oligomer.
Prucnal, et al., U.S. Pat. No. 3,874,906 teach a method of applying and curing a polyester-acrylate containing coating composition comprising adding N-vinylpyrrolidone to the composition and subsequently applying the composition to a substrate and subjecting it to actinic light to cure. Lorenz, et al., U.S. Pat. No. 4,129,709 disclose a coating composition comprising an oligomer produced by reacting polytetrahydrofuran with a diisocyanate, N-vinyl-2-pyrrolidone and an acrylic acid ester having a boiling point of at least 200.degree. C. at 760 mm Hg.
Priola, et al., U.S. Pat. No. 4,348,427 teach a method of coating surfaces by applying to the surface to be coated, a mixture composed by at least one compound such as an epoxy-acrylate resin, a polyester alpha-omega-acrylate resin, an unsaturated polyester resin, or a urethane-acrylate resin and by at least one unsaturated compound of the amide, lactam, piperidone and urea classes and subsequently irradiating the coated surface with radiation in the range of 200 to 400 nm.